This invention relates generally to maintenance of polishing pads for semiconductor wafers and more particularly to an apparatus and process for reconditioning polishing pads to maintain flatness.
Semiconductor wafers are generally prepared from a single crystal ingot, such as a silicon ingot, which is sliced into individual wafers. Each wafer is subjected to a number of processing operations to facilitate the installation of integrated circuit devices and to improve their yield, performance, and reliability. Typically, these operations reduce the thickness of the wafer, remove damage caused by the slicing operation, and create a flat and reflective surface. Chemical-mechanical polishing of semiconductor wafers is one of these operations. It generally involves rubbing a wafer with a polishing pad while dispensing a slurry containing an abrasive and chemicals, such as a colloidal silica and an alkaline etchant, to produce a surface that is extremely flat, highly reflective, and damage-free.
The polishing pad is circular or annular in shape and has a polishing surface (i.e., that portion of the surface area of the pad which contacts and polishes the wafer) which must be extremely flat to produce wafers that are likewise flat. Unfortunately, polishing surfaces can acquire an uneven shape after use. In a conventional semiconductor wafer polisher, the wafer is held with force by a polishing arm against the rotating polishing pad. The polishing arm may also move the wafer across the polishing pad in an oscillatory fashion as the pad rotates. After a number of polishing cycles, pressure and heat on the polishing pad cause variations of pad shape in a central annular region of the pad that contacts the wafer. Further, the polishing surface becomes worn in the central annular region. Thus, the cross sectional profile of the polishing surface of the pad becomes nonplanar.
Slurry particles typically become deposited on polishing pads and further degrade polishing effectiveness. A typical polishing pad is made of a polyester felt impregnated with polyurethane resin. During a polishing process, particles from the slurry and reaction products become adhered to fibers in the pad. When the pad becomes soaked with slurry particles, its polishing ability is reduced. The particles can be unevenly distributed across the polishing surface, making the surface irregular. The combination of pad wear and slurry deposition can make the pad either concave or convex.
Accordingly, polishing pads must be periodically reconditioned, or dressed, to restore a flat cross sectional profile and scrape away deposited slurry particles. One way reconditioning is accomplished is by using a dressing wheel which carries abrasive material on a pad shaping surface of the wheel. The wheel is held in position over the polishing surface with its pad shaping surface engaging the pad. The wheel is restricted from rotating about the axis of rotation of the pad as it turns, but may be permitted to freely rotate about its own center. The pad shaping surface rubs against the pad, abrading away the thicker portions so the profile is made more flat. Specifically, the inner and outer edge margins of the polishing surface are lowered to the level of the central region by abrading away the inner and outer edge margins. The tool also removes deposited particles of slurry. The abrasive material on the tool, such as diamond, is disposed in a continuous ring located at the periphery of the tool.
The pad shaping tool can become slightly misaligned with the polishing pad resulting in uneven shaping. The tool is typically held by a connector at its center to a fixture generally above the pad shaping surface. The fixture holds the pad shaping surface generally parallel the pad, and has a bearing that engages the connector and permits the tool to rotate about its center. As the pad moves relative to the abrasive material of the pad shaping tool, frictional force from the pad drives the tool to move along with the pad. The fixture opposes the force and holds the tool from translating with the pad. However, the force creates a moment about the bearing and the fixture, since they are spaced above the point of force application. The moment urges the tool to pivot about a point at the connector between the tool and the fixture.
Mechanisms for attaching tools, such as the bearing and the fixture, often have some degree of flexibility and looseness that allows a finite movement when opposing forces or moments. The moment from the frictional force induces a deflection in the fixture so that the tool pivots a small angle and is no longer in horizontal alignment with the pad. A leading edge of the abrasive surface of the tool (i.e., the side of the wheel that first contacts the central annular region of the rotating pad) is pushed relatively more into the pad, while the trailing edge of the tool (i.e., the side of the wheel that last contacts the rotating pad) is pushed relatively less into the pad. After sufficient abrasion to an equilibrium, the pad shape should conform with the shape of the abrasive material at the tool peripheral. However, because of pivoting, the cross sectional profile of the polishing surface becomes concave.
Thus, the frictional force creates a moment that pivots the tool and tends to make the pad profile concave. The tendency yields uncertain results, and operators have devised various cumbersome procedures for reconditioning pads that vary depending on the initial profile (i.e., whether convex or concave). For instance, when a pad is convex the wheel may be allowed to rotate but when a pad is concave the rotation of the wheel is restricted. These procedures often yield non-repeatable results and may require trial and error to obtain polishing pads that are flat.